The invention relates to a method for determining the pelvic inlet plane of the pelvic bone, which is defined by the following three points of the pelvic bone:    point A: spina iliaca anterior superior left    point B: spina iliaca anterior superior right    point C: symphysis pubisby non-invasive determination of the position of one of the two points A or B and point C. The invention also relates to an apparatus for performing this method.
When inserting implants and performing other operations, it is often necessary to describe the anatomical conditions in an abstract manner in order to record the anatomical conditions in mathematical models, for example, in connection with known navigation systems and with the processing of the thereby obtained data in data processors.
For example, it is known (DE 10 2005 003317 A1) to describe the pelvic bone by defining a pelvic inlet plane, which is defined by three prominent points of the pelvic bone, namely the following points:    point A: left spina iliaca anterior superior (left anterior superior iliac spine)    point B: right spina iliaca anterior superior (right anterior superior iliac spine)    point C: symphysis pubis (pubic symphysis).
For example, these prominent points may be percutaneously palpated. It is also possible to non-invasively determine these points in some other way, for example, by an ultrasonic sensor, by X-rays or by other imaging methods, for example, computed tomography methods.
In all cases, the prominent points can be used to describe the pelvic inlet plane and, therefore, this pelvic inlet plane can be used to calculate the position of the pelvic bone in subsequent computations.
However, a precondition for this method is that the three prominent points be accessible, for example, during percutaneous palpation or when a preferably navigated ultrasonic sensor is used. However, this is not possible in all cases, for example, when a patient is lying on his side on an operating table only one of the points A or B is accessible, i.e., either the left or the right spina iliaca anterior superior only. It is, therefore, impossible to determine the pelvic inlet plane in this special position of the patient.
An object of the invention is to indicate a method by means of which, in these cases, too, where one of the points A and B is not accessible, the pelvic inlet plane can be determined.